Pipettes used for liquid dosage in laboratories comprise a piston movable in a cylinder and serving to aspire liquid into and to dispense liquid from a tip joined to the set of a cylinder according to a selected pipetting function. The liquid volume is usually adjustable. There are also electronic pipettes whose piston is actuated by means of an electric motor and an associated control system. Electronic pipettes have a control system and an associated user interface for setting i.a. the volume and the other necessary pipette functions and for giving commands for performing operations. The user interface has the necessary keys for this. The user interface also comprises a display, by means of which i.a. the volume and any other necessary data can be displayed. The display can also show menus allowing selection of functions and input of settings by means of the keys. When the desired function has been selected and the volume and other settings have been entered, depression of the operating switch automatically carries out the following step of the function until the entire function has been completed.
The pipetting functions comprise e.g. direct, reverse, step and mixed pipetting. Direct pipetting involves aspiration of a desired volume into the pipette and discharge of the volume. Reverse pipetting function involves aspiration of a volume greater than the one desired into the pipette, with the desired volume being subsequently dispensed. Step pipetting involves aspiration of a volume into the pipette, the volume being subsequently dispensed in a plurality of minor portions. Mixed pipetting comprises discharge of liquid into a receptacle containing a liquid, the pipette tip being located under the liquid surface and the piston performing several reciprocating movements during the discharge in order to mix the liquids in the receptacle.
Such a pipette is e.g. Finnpipette® BioControl (manufacturer Thermo Electron, Finland).
Some pipettes also have a separate button for interrupting the step function. When this button is depressed during dosing, the entire liquid amount in the pipette is dispensed in one single pass (i.e. not in minor portions as in step pipetting). In some pipettes, the step function can be interrupted by depressing the operating switch long enough.